The Urban Dictionary T-Shirt

An inarticulate expression of one's inadequacy or of feeling awkward, which is then projected as an insult onto someone else.

If you hear people say this around you, they are probably feeling awkward and in turn calling you awkward or "deet" as a form of Freudian projection. A proper response would be to tell them to: "learn how to not feel awkward" or more eloquently "grow the fuck up".